The first US 15-A was born in 1936 as a renumbering of US 401 from the South Carolina State Line below Laurinburg to Raleigh and NC 13 from Raleigh to Creedmoor. The route continued as a duplex with US 15 south all the way to Walterboro, SC.

US 401 Laurinburg-Fayetteville-Raleigh

US 15-A Laurinburg-Fayetteville-Raleigh

Back to US 401 Laurinburg-Fayetteville-Raleigh

Specifically, 15-A follwed US 15-401, then US 401 Business through Laurinburg out to US 401.
US 15-A followed US 401 to Raeford, except it crossed the Lumber River slightly north of the current causeway. 15-A used US 401 Business through Raeford, then picked up 401 again. US 15-A followed US 401 to the Owens area except it used the old 401 alignment at the Fayetteville Fish Hatchery.
US 15-A used US 401 Business into Fayetteville, but stayed on Raeford Rd all the way in to Broadford, then north to Hat St then east into central Fayetteville. US 15-A turned north at Ramsey St and headed out of town as US 401 Business does now.
US 15-A followed US 401 through Lillington to Fuquay-Varina except it used Chalybeate Rd through Chalybeate Springs. North of F-V, US 15-A followed today's US 401 until just north of the US 70/NC 50 jct, where US 15-A originally stayed on Glendale St and Saunders St north to W. Cabarrus St, then east to Dawson St then north to W. Jones St, then east to Halifax just north of the Capitol Square. US 15-A followed Halifax to W. Peace St west to Glenwood Ave north to NC 50.
US 15-A followed NC 50 to Creedmoor. 15-A may briefly have used the Old Creedmoor Rd that swings west to nearly the Durham County line by Rogers Store (1935 maps show NC 13 using it while 1936 Wake County map shows current NC 50's routing). in Creedmoor, US 15-A ended at US 15 at the NC 56 jct.
In 1937, US 15-A was rerouted in Raleigh to leave Saunders at South St east to Fayetteville St north to Morgan St east to Wilmington St north to W. Peace St west to Glenwood Ave.

US 15-A original Raleigh routing

US 15-A on its second Raleigh routing

Between 1939-44, US 15-A was placed on the current US 401 around Chalybeate Springs.
In 1947 or 1948, US 15-A was rerouted in Raleigh to use Edenton St west from Wilmington, then north on Halifax to Peace St west.
About 1950, US 15-A was rerouted off of Glendale altogether and instead used Wilmington St north all the way to central Raleigh. US 15-A also split into one-way alignments around the Capitol area, with northbound using Wilmington-Edenton-Halifax while southbound used Halifax, Edenton west, then Salisbury St south to South St east to Wilmington St SB.

US 15-A on its third Raleigh routing

US 15-A on its fourth Raleigh routing

Although S.C. Officials from 1950 forward show the US 15-15A duplex gone, NC Officials didn't stop showing it until 1952 or 1953. I do not understand what the intent of this duplex was. However, it made US 15A one of (if not) the longest 'A' routes ever at about 300 miles.

US 15-15A duplex

In 1954, US 15-A was rerouted around the north and west of Raeford on current US 401's routing. The old route through town was not numbered as anything for at least 3 years but is now US 401 Business.
In 1956, the entire US 15-A from Laurinburg to Raleigh became US 401, while Raleigh to Creedmoor became NC 50. Around that time, the route in Fayetteville changed to use Robeson St from Raeford Rd. It is unclear if 15-A did this or if only US 401 did.

NC 13 north of Raleigh

US 15-A north of Raleigh

NC 50 north of Raleigh

There was also a US 15-A TRUCK route explicitly shown on Cumberland County Maps beginning in 1949. US 15-A Truck used Broadford south from Raeford Rd, then east along Russell St to Gillespie St.

US 15-A's route through Fayetteville

US 15-A TRUCK route appears in Fayetteville

The 2nd US 15-A was born in 1953 as a renumbering of mainline US 15-501 through Chapel Hill when US 15-501 was placed on the eastern bypass of Chapel Hill. US 15-A was renumbered as US 15 Business in 1960. Today it is part of NC 86 and also Franklin St.

US 15 through Chapel Hill

US 15A-501A through Chapel Hill

US 15-501 Business replaces 15A-501A.

The 3rd US 15-A was born in 1956 or 1957 when US 1-15-501 was put on a western bypass of Sanford. In 1960 this was renumbered as US 1-15-501 Business. Today it is just US 1 Business and below NC 42 just Carthage St.